1. Field of the Invention
The present invention relates to a membrane switch used in various electronic devices, incorporating a light illumination of a key top portion by a light emission diode (LED).
2. Description of the Background Art
As a switch unit to be used in various electronic devices, there has been a light illumination membrane switch which incorporates a light illumination of key top portions by LED for the purpose of easier recognition of the key top portions even in a dark environment.
An exemplary configuration of such a conventional light illumination membrane switch is shown in FIG. 1A, which comprises: a surface sheet 2 with key top portions 1 for forming numerical keys thereon; a translucent diffuser member 3 attached below the surface sheet 2; and a printed circuit board 6 having clicking diaphragm spring members 4 located below the key top portions 1, LED chips 5 provided in vicinities of the clicking diaphragm spring members 4, and connection terminal 6' for connecting electrode circuit members (not shown in FIG. 1A) provided below the clicking diaphragm spring members 4 to an external circuits. These surface sheet 2, the translucent diffuser member 3 and the printed circuit board 6 are assembled together to form a thin layered structure as shown in FIG. 1B.
Here, the translucent diffuser member 3 is made of a silicone rubber, and as shown in FIG. 2, this translucent diffuser member 3 has a plurality of concaved portions 4a for housing the clicking diaphragm spring members 4 and a plurality of concaved portions 5a for housing the LED chips 5, on its back side facing toward the printed circuit board 6 in correspondence to the clicking diaphragm spring members 4 and the LED chips 5 provided on a front surface of the printed circuit board 6.
A key section and a light source section of this conventional light illumination membrane switch have a detailed configuration as shown in FIG. 3, in which the printed circuit board 6 comprises an insulative base layer 6a, electrode circuit members 6b, an adhesive layer 6c, and a cover lay polyimide film 6d, all of which are layered together, and the cover lay polyimide film 6d of this printed circuit board 6 is attached to the translucent diffuser member 3 by an adhesive layer 7.
The clicking diaphragm spring members 4 make electrical contacts with the electrode circuit members 6b in response to the depression of the key top portions 1, as the clicking diaphragm spring members 4 located below the key top portions i are convexed downwards in response to the depression of the key top portions 1, and the electrical contacts between the clicking diaphragm spring members 4 and the electrode circuit members 6b can be broken when the depression of the key top portions 1 is released as the clicking diaphragm spring members 4 resume the original upwardly convexed forms, so as to realize an ON/OFF switching function.
The LED chips 5 are commercially available premanufactured chip components, and each of the LED chips 5 has a detailed configuration as shown in FIG. 4, which comprises a substrate 9 made of ceramic such as alumina or resin such as glass epoxy, a LED bare chip element 10 mounted on an upper surface of the substrate 9, an electrode member 11 formed over the substrate 9, a bonding wire loop 12 for electrically connecting a terminal at a top of the LED bare chip element 10 and the electrode member 11, and a translucent diffuser resin 13 covering over the LED bare chip element 10 and the bonding wire loop 12 completely, where this LED chip 5 is mounted on the printed circuit board 6 through conductive soldering 14 such that the electrode circuit member 11 is electrically connected with the electrode circuit members 6b through this conductive soldering 14.
Now, a reduction of a thickness of such a conventional light illumination membrane switch has been limited by the height required for the light source section incorporating the LED chip 5. Namely, the total height of the LED chip 5 is normally about 1.2 mm, and when this LED chip 5 is soldered on the printed circuit board 6, the LED chip 5 is elevated above the electrode circuit members 6b by the conductive soldering 14 for about 0.1 mm. Furthermore, for the sake of the sufficient light illumination from the LED chip 5, it is necessary for the translucent diffuser resin 13 to have a thickness of about 0.3 mm above the LED bare chip element 10. For these reasons, a conventional light illumination membrane switch inevitably has the thickness of at least about 1.6 mm and further reduction of the thickness of a light illumination membrane switch has been practically impossible.